1. Field of the Invention:
In recent years, as notebook computers have become more and more compact and lighter in weight so that they can be easily portable and used almost anywhere, a liquid crystal display has been considered a very important screen display device for holding display data. One conventional display device for holding display data in units of pixels is sometimes referred to as a TFT (Thin-Film-Transistor) active matrix type liquid crystal display device or TFT LCD device which has many pixels arranged in a matrix. A color pixel is formed usually by combining three pixels. However, the size of a conventional TFT LCD device is relatively large when used for the screen display of a notebook computer. In view of its large size, the conventional TFT LCD device can only be fabricated on a piece of large glass substrate with polysilicon thin-film transistors.
As a consequence, there has been developed in the last few years a new class of mini-displays which are based on a single crystal silicon substrate. These newer mini-displays can be manufactured using current CMOS technological processes, which can provide better yield and a higher level circuit integration than the existing TFT LCD devices. These mini-displays are referred to as Silicon-Chip-Display (SCD) devices, which are in essence miniature versions of the TFT LCD device. In fact, the LCD portion of the mini-display is quite similar to that of the TFT LCD device, but is made on a much smaller scale, e.g., on top of a silicon chip. The image on top of the mini-display is typically magnified for viewing by an optical system. Dependent upon the particular application, the optical system may be quite complex.
The specific applications for the SCD devices can be generally classified into three major categories. Firstly, the current important application is the one for use in the area of very large screen projectors in which the size of the display device is on the order of thirty inches or more. Secondly, there is an application for utilization as a desktop computer monitor, where the size of the display device is in the range between seventeen and twenty-one inches. Lastly, the third application for the SCD devices is for use as a portable personal display unit.
2. Description of the Prior Art
There is shown in FIG. 1 a simplified perspective view of a conventional Silicon-Chip-Display (SCD) device 10, labeled xe2x80x9cPrior Art,xe2x80x9d which includes a piece of silicon chip substrate 12, a plurality of bonding pads 14 disposed on its peripheral edges, and a display cell array 16 located in the central part of the substrate 12. A circuit area 18 is positioned around the cell array 16. A top glass cover 20 having a seal-ring 22 is securely mounted over the display cell/the circuit area with a liquid crystal (LC) layer sandwiched therebetween. Typically, the silicon chip substrate 12 has a dimension of less than 20 millimeters on each of its sides. Unlike the prior art TFT-LCD display being operated in the Transmissive Mode, the SCD device is operated in the Reflective Mode due to the fact that the silicon substrate is not transparent to light.
In FIG. 2, there is illustrated a schematic circuit diagram of a conventional SCD cell 24 which is comprised of an access transistor T1a having a gate, drain, and source and a large storage capacitor Cs. The gate of the access transistor is connected to a wordline WL. The source of the access transistor is connected to a bitline BL, and its drain thereof is connected to one end of the storage capacitor Cs and to a mirror plate 26. The other end of the capacitor is connected to a ground potential. A top electrode 28 is formed in a spaced-apart relationship to the mirror plate 26 with an LC layer 30 sandwiched therebetween. The mirror plate 26 functions optically as a light reflective mirror as indicated by arrows 29,31 and operates electrically as a LC modulator.
The cell structure 24 of FIG. 2 is sometimes referred to as a DRAM-type cell due to the fact that it resembles closely the one-transistor cell of a conventional DRAM device. However, there are two major factors which render the SCD cell structure to be smaller than the conventional TFT-LCD display device. First, a gate insulating film used for forming the storage capacitor is extremely thin and thus the surface area thereof can be made smaller than the conventional TFT device. Secondly, since the SCD cell is operated in the Reflective Mode rather than in the Transmissive Mode the access transistor can be fabricated at any location under the mirror plate without causing a blockage of light. Further, the SCD cell structure has the advantage of being able to utilize the current CMOS silicon technology so as to fabricate integrated drivers and other integrated circuits onto the silicon substrate, thereby achieving a more compact and highly reliable display device. This cell structure is also called a 1C1M (one-capacitor one-mirror) cell.
In order to generate a color image from the SCD display device, there are various known techniques used dependent upon the type of application. For example, in a large screen projection type color display device there are always used three SCD devices together with precision optics so as to process the three colors, corresponding to red (R), green (G), and blue (B). On the other hand, in a portable type display device where there is a concern for size, weight, and/or cost only one SCD device is used on which is a color image must be generated. In order to achieve the color image for the portable display, there are used three-pixel cells within the SCD device which is then covered with color filters for the corresponding RGB colors. However, the pixel array area becomes then approximately three times larger which is unsuitable for a high yield on the CMOS silicon technology. In addition, the use of color filters makes the standard CMOS silicon process more complex and thus increases cost.
In order to overcome these disadvantages, there have been developed a prior art technique of creating a color image in a one-pixel cell which is referred to as a xe2x80x9cField Sequentialxe2x80x9d (FS) method. This FS method writes RGB data to each of the one-pixel cells in the pixel array in three sequential operations at three times the clock rate. During each of the three sequential operations, a corresponding RGB light source is activated synchronously.
This FS method will function acceptably as long as the response time of the liquid crystal LC is sufficient enough. If this is the case, then the three sequential colors will be effectively combined into a single color image. Therefore, the effective LC response time for each of colors is ⅓ of the frame time reduced by the amount of time it takes to write a color field. However, a serious problem arises when this FS method is utilized in a relatively large display device. This is because there may not be enough time for the LC to respond. For instance, a standard 60 frames/sec video signal has a total time of 16.67 ms for displaying a frame. In the case of the three-pixel cells where the RGB colors can be processed in parallel, each color has a full frame time so as to process and then display the video data. On the other hand, in the FS operation each of the colors has only ⅓ of the frame time or 5.56 ms in order to write data to the storage capacitors, to then wait for the LC to respond, and to then finally strobe the pixel array with the corresponding RGB light source. The length of time for light strobing (LS) the pixel array will determine the bright of the color image. Consequently, in order to achieve a high quality video in the SCD device operated with the FS method, it has become necessary to effectively increase the LS time.
Accordingly, it is still desirable to provide an improved SCD cell structure operable in a field sequential mode for use in a liquid crystal devices. Further, it would be expedient that the SCD cell structure include circuit means for increasing effectively the length of the LS time. This is achieved in the present invention by the provision of a novel SCD cell structure which includes multiple storage capacitors for storing video data prior to corresponding color-field times for display.
Accordingly, it is a general object of the present invention to provide an improved SCD cell structure operable in a field sequential mode for use in liquid crystal devices which overcomes the problems of the prior art display devices.
It is an object of the present invention to provide an improved SCD cell structure operable in a field sequential mode for use in liquid crystal devices which has a better yield and a higher level circuit integration than existing display devices.
It is another object of the present invention to provide an improved SCD cell structure operable in a field sequential mode for use in liquid crystal devices which has a higher quality color image than conventional display devices.
It is still another object of the present invention to provide an improved SCD cell structure operable in a field sequential mode for use in liquid crystal devices which includes circuit means for increasing effectively the length of the light strobing time.
In accordance with a preferred embodiment of the present invention, there is provided a Silicon-Chip-Display (SCD) cell structure operable in a field sequential mode for use in liquid crystal display (LCD) devices and a method for operating the same provided. The SCD cell structure is formed of first through third write-enable transistors, first through third storage capacitors, and first through third display-enable transistors. Each of the write-enable transistors is sequentially turned on so to pre-load video data into the corresponding storage capacitors during one color-field time prior to when each of the corresponding display-enable transistors is sequentially turned on for displaying an associated color-field. As a consequence, there is rendered a higher quality video image by allowing the LC response time and/or the light-strobing time to be increased.